SOx produced from the burning of organic sulfur-containing compounds present in fuel oils not only can cause acid rain, but also can poison irreversibly the three-way catalysts in the tail gas cleanup systems of engines. Therefore, it has been receiving much attention. However, sulfur-containing compounds in fuels, especially gasoline, are actually very difficult to remove. The conventional hydrodesulphurization (HDS) method requires severe conditions (e.g., high temperature, high pressure, and high hydrogen consumption), which result in high operating-cost and a substantial octane loss, particularly for FCC gasoline. Therefore, it is highly desirable to develop non-HDS methods to meet the demands of producing clean gasoline with an extremely low sulfur level. Recently, oxidative desulphurization (ODS) is considered to be one of the most promising alternative processes due to the mild operation condition and no consumption of expensive H2. However, thiophene, one of the main sulfur-containing compounds in FCC gasoline, is most difficult to be removed with the conventional oxidative desulphurization. As an energy-saving and environmentally friendly desulphurization process, photocatalytic oxidation desulphurization using molecular oxygen as the oxidant has received increasing attention in recent years.
F. Berthou (F. Berthou and V. Vignier, Int., J. Environ. Chem., 1986, 27, 81.) tried to desulfurize directly under sunlight, DBT (Dibenzothiophene) was dissolved in seawater and authors achieved a desulfurization rate of 80% after 8 days. However, the time of that process was too long and this process was not practical. bdelwahab and Gaber used anatase TiO2 as the photocatalyst for photocatalytic oxidation of DBT in acetonitrile, they also analysed the reaction products (A. M. A. Abdel-wahab, A. E.-A. M. Gaber, J. Photochem. Photobiol. A, 1998, 114, 213.). Robertson and Bandosz prepared titanium (IV) oxide-hectorite nanofilm photocatalyst on quartz slides and used it to photooxidize DBT in tetradecane, which was used as a model for diesel fuel (Robertson and andosz, Applied Catalysis B: Environmental, 2004, 51, 195.). Matsuzawa et al. studied photocatalytic oxidation of DBT and 4,6-dimethyldibenzothiophene in MeCN using TiO2 (S. Matsuzawa, J. Tanaka, S. Sato, T. Ibusuki, J. Photochem. Photobiol. A, 2002, 149, 183.). To lower the cost of the oxidant, they also tried to use oxygen to substitute H2O2.
Yasuhiro Shiraishi et al. (Yasuhiro Shiraishi et al., Ind.Eng.Chem.Res. 1996, 35, 586; Yasuhiro Shiraishi et al., Chem. Commun., 1998, 2601 ; Yasuhiro Shiraishi et al., Ind.Eng.Chem.Res., 1999, 38, 310; Yasuhiro Shiraishi et al., Ind.Eng.Chem.Res. 2000, 39, 1345; Yasuhiro Shiraishi et al., Journal of Chemical Engineering of Japan, 2002, 35, 489.). did many researches in the photosensitization of DBTs, they used the photosensitizers such as benzophenone and 9, 10-dicyanoanthracene (DCA). Jincai Zhao et al. used 2-(4-methoxyphenyl)-4,6-diphenylpyrylium (MOPDPP+BF4−) as the photosensitizer to photooxidize DBT to DBTO and DBTO2 in CH3CN 100% in 10 h (Jincai Zhao et al., J. Phys. Chem. B, 2006, 110, 2942.). But the photosensitizer is easy to be oxidized and difficult to be recovered after reaction. To improve the desulphurization efficiency furthermore researchers use other assistants such as ultrasound irradiation and γirradiation to photooxidize DBT (Y usufg et al., Environ. Sci. Technol., 2005, 39, 8557; Naiqiang Yan et al., Energy & Fuels, 2006, 20, 142; Naiqiang Yan et al., Journal of Chemical Industry and Engineering (China), 2003, 54, 1279.).
It is obviously found from the prior part that the catalysts used in the photocatalytic oxidation desulphurization have some disadvantages: (1) The activity of catalyst needs to be further improved; (2) The photosensitizer is easy to be oxidized and difficult to be separated, recovered after reaction; and (3) Need other assistants and the operation cost is relatively high. Therefore, it is urgent to develop the visible-light-responsive semiconductor photocatalyst for photocatalytic oxidation desulphurization under visible light irradiation, which is environmentally friendly and easily recovered.